It should be remembered that 1938 had been a difficult year for Tolkien. While he had written much for his Hobbit sequel, he had suffered through deep depression in August and a nasty flu in December. Tolkien had also just finished the first several writing phases—as his son Christopher has labeled them—of what would become The Fellowship of the Ring, when he began research and thought regarding his proposed lecture, “On Fairy Stories.” He had hoped to deliver a paper on the same topic to an undergraduate society at Oxford in 1938, but that had fallen through.[3] This would be his chance to rectify that, and with the added benefit of serious academic legitimacy. On the evening of March 8, 1939, Tolkien delivered his lecture at the University of St. Andrews.

To state that the lecture was important to Tolkien and, frankly, to the world of literary criticism, would be a gross understatement. Coming when it does in Tolkien’s writing career, “On Fairy Stories” reveals more about the mind and soul of the man than any other non-fiction work he produced throughout his lifetime. It is, to be certain, seminal and beautifully so. Like his own Stoic and mystical understanding of Faerie, his talk was, in turns, excellent, insightful, and brilliant. It also offers, at its most fundamental level, a counterrevolution of ideas, an image of the world directly counter to that held by the fascists, communists, and ideologues of all varieties of the twentieth century
— Read on theimaginativeconservative.org/2020/10/tolkien-on-fairy-stories-setting-bradley-birzer.html